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15-381 Artificial Intelligence
Means-Ends Analysis andConstraint Propagation
Jaime Carbonell4 September 2001
Topics Covered:
Means-Ends Analysis
Search Control Rules in MEA
Constraint-Based Search
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Searc
Planning:
Para!eteri"ed #perations
M$lti-State Transitions
nstead o!" #pi$%" Si S %$ &e ha'e #p($l" )S( *) Sl*
Preconditions and Post-Conditions Con%uncti'e set o! !irst-order predicates
Ar+uments can be constants or ,typed 'ariables
ntentional description o! subset o! all states
Pre-image )S( * states .here preconditions are true Post-image )S1* states .here post-conditions are true
Re/uires Consistent 'ariable bindin+s .ithin andacross preconditions and post-conditions
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Searc
Planning:
Para!eteri"ed #perations
%irst E&a!ple
#ERA#R R3E-CAR,car5$ dri'er5$ (eys5$ loc-15
6RE" ,A car5 loc-15
,A dri'er5 loc-15,C#7A7S-8AS car5
,9A3E (eys5 dri'er5
,C#RRES#7 (eys5 car5:
6#S" ,A car5 loc-25,A dri'er5 loc-25
,7# ,A car5 loc-15
,7# ,A dri'er5 loc-15::
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Searc
Planning:
Para!eteri"ed #perations
Second E&a!ple
,re'ious e;ample"
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Searc
Planning:
Para!eteri"ed #perations
Interpretation
A plan is an o-path" S0 !ollo.ed by a se/uence o!
instantiated operators .hich result in the +oal state
3ariables match ob%ects in state o! specified types
only !or .hich te preconditions old at plan
e;ecution time
lannin+ can proceed by !or.ard or bac(.ard ,orany other search method
More on lannin+ !rom 3eloso ,later lecture
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Means-Ends Analysis
'ac(caining)S$*goaling Searc1
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Control +$les for MEA
Coice Points in MEA Choose #perator$ i! se'eral applicable
Choose 8oal$ i! 5 1 sub+oals pendin+
Choose 3ariable Bindin+s$ i! 5 1 tuple
Types of Control +$les Select I choose an alternati'e
and eliminate other contenders
Reject I Re%ect an alternati'e
and retain other contenders
Prefer I ry one alternati'e !irst
and retain others !or possible bac(trac(in+
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Control +$les for MEA
E&a!ple
C#7R#
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Constraints
E&a!ple
Lind a .ay to !it components ,1$2$$4 into slots,A$B$C$ such that" Each slot only ta(es one component
Slots are in
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Constraints
,east Co!!it!ent Metod
1 Lor each 3ariable !ind all le+al unary-constrainedassi+nments
2 ! no assi+nments possible$ return LA
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Constraint-'ased Searc
A )1$2$$4*
B )$4*
C )1$2$$4*
)$4*
A )1$2$4*C )1$2$4*
)4*
A )1$2*
C )1$2*C )2*
A )1*
SCCESS
A )1$2$*
C )1$2$*
)*
LA
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.is/Advantages of Constraints
Reduce the search space Early !ailure ,upon constraint 'iolation
8enerate minimal-uncertainty step ,least
commitment strate+y
#nly applicable to satis!iability problems Linds an ans.er$ not necessarily optimal
7ot all problems can be cast as constraints to
satis!y